Twenty-year follow-up of functional treatment with a bionator appliance: A retrospective dental cast analysis

Class II malocclusion is a common finding among orthodontic patients and an important medical indication for orthodontic treatment.¹  The bionator according to Balters is a functional orthopedic appliance to treat Class II malocclusion in growing patients with skeletal harmony of the jaws or when skeletal harmony can be expected due to an inherited favorable growth potential and direction of the upper and lower jaw.^2,3  This kind of appliance is especially applicable in patients with orofacial dysfunctions such as persisting sucking habits or habitual mouth breathing and is supposed to harmonize tongue, lip, and cheek muscles.²  Ascher modified Balters' bionator by adding an acrylic lower incisor capping to prevent the lower incisors from protruding.⁴  Although bionator treatment during prepuberty mainly causes dentoalveolar adaptation, skeletal effects are possible during pubertal growth.^5–8  It seems that bionator treatment displaces the mandible anteriorly but limits its normal counterclockwise rotation by changing the direction, but not the amount of condylar growth.⁹  Nevertheless, upper incisor retroclination and lower incisor proclination are to be expected as side effects despite the capping of the lower incisors.² 

Long-term stability of treatment results is a key to successful orthodontic treatment. Additionally, it is essential to understand the difference between relapse after treatment and physiological processes related to aging.^10–12  Several studies have evaluated cephalometric and morphometric long-term changes after bionator therapy followed by fixed appliances.^5–7,13,14  Francisconi et al. reported that therapy with a bionator followed by fixed appliances produced favorable results in apical base and molar relationships, overjet, and overbite that remained stable 10 years post-treatment.¹⁵  Only a few studies, however, investigated long-term changes of dental arches after functional treatment more than 15 years after treatment. There is no study assessing long-term stability of bionator therapy that was not supplemented by fixed appliances. Therefore, the aim of the present study was to assess treatment effects and their long-term stability of Class II bionator therapy with the Balters bionator modified by Ascher based on patients' dental casts before, directly following, and 20 years after treatment.

Orthodontic records of patients treated with a Balters bionator modified by Ascher^2,4  were retrospectively collected in 2018 from the archives of the Department of Orthodontics, University Medical Centre Munich, Bavaria, Germany. Bionator treatment was initiated between 1973 and 1979 and exclusively conducted by one experienced orthodontist (IR). Mean patient age before treatment (T0) was 9.8 years (SD: 1.5), after bionator treatment (T1) 13.3 years (SD: 1.9) and after 20 years of follow-up (T2) 33.3 years (SD: 2.3). The facial type of each patient was evaluated according to Individualized Cephalometrics with a ceph template and a harmony box according to Segner and Hasund (retrognathic <79°, orthognathic 79°–85°, prognathic >85°).¹⁶  The following eligibility criteria were applied: (1) treatment with a Balters bionator modified by Ascher,^2,4  (2) no treatment with fixed appliances at any time, (3) skeletal Class II defined as ANB > 4° in an orthognathic facial type and ANB > 2° in a retrognathic facial type,¹⁶  (4) available dental casts at all time points: T0, T1, and T2, (5) no orthodontic extraction therapy, space closure, or opening. Orthodontic records were collected for treatment purposes at the start (T0) and end (T1) of treatment. In addition, long-term assessment was performed 20 years after the end of treatment (T2). Ethical approval was obtained from the ethics committee of the University of Munich (77/97) and patient consent for the data collection at T2 was also obtained. From T1 to T2, patients received no orthodontic treatment or retention.

Alginate impressions (Kaniedenta, Herford, Germany) were taken of the upper and lower dental arch and a wax construction bite of the therapeutically desired jaw relation was made without disrupting patients' postural resting position in the vertical plane (lip closure without muscular activity). The latter was taken in the sagittal plane in an edge-to-edge incisor relationship and in the vertical plane with or without a slight vertical opening. In the transverse plane, the mandibular midline was positioned to coincide with the maxillary and facial midlines. The bionator appliance consisted of an acrylic monobloc with extensions as small as possible, a palatal bar (coffin spring), and a buccinator loop. Following Ascher's modification, the lower incisors were covered with acrylic with slight impressions of the upper incisors for stabilizing them vertically (Figure 1). If necessary, bionators were adjusted to raise the bite. Patients were asked to wear their appliances 24 h/d apart from eating and toothbrushing. Actual usage times were self-recorded by the patients on adherence cards. As soon as Class I at the first molars and overjet reduction were evident clinically, wearing time was reduced to 12 h/d (night).

Measurements on patients' dental casts of the upper and lower dental arches were undertaken by one experienced orthodontist (RJ). A digital caliper (HSL 246-15, Hammacher, Solingen, Germany) was used with a precision of 0.01 mm. All data were, however, rounded to the first decimal prior to statistics. All outcome variables measured are listed in Table 1, corresponding to standard parameters in orthodontic dental cast analysis and characterizing the dental arches, their relationship, as well as the degree of crowding (Little's irregularity index),¹⁷  and malocclusion (PAR index).¹⁸ 

To determine intra- and interrater reliability, 30 casts were selected randomly and measurements performed a second time after a time interval of 4 weeks by the same (RJ) as well as a different experienced investigator (VK). To calculate casual and systematic errors, Dahlberg's formula and the intraclass correlation coefficient (ICC; two-way mixed, absolute agreement) were used.

Data were analyzed using the software IBM SPSS Statistics 23 (IBM, Armonk, NY, USA). Mean (M) and standard deviation (SD), as well as median (MD) and interquartile range (IQR) were reported as descriptive statistics. More than 5% of the data showed a non-normal distribution. Therefore, nonparametric Friedman's two-way analysis of variance by ranks was performed and pairwise comparisons calculated with Dunn's post hoc tests. A P value of ≤.05 was considered statistically significant. Effect sizes were calculated as Pearson's r and interpreted according to Cohen.¹⁹ 

Based on the inclusion criteria, 18 patients (nine males and nine females) and their corresponding dental casts at T0, T1, and T2 were available for analysis (Figure 2). A total of 134 available patient records with bionator treatment were assessed for eligibility. 98 patients were excluded due to nonparticipation in the follow-up study, while 18 of the remaining 36 patients needed to be excluded due to not meeting the inclusion criteria. Baseline characteristics of the 18 included study patients are displayed in Table 2. According to Individualized Cephalometrics by Segner and Hasund,¹⁶  61.1% (n = 11) had a retrognathic facial type, and 38.9% (n = 7) were orthognathic. At the end of treatment (T1), the average age was 13.3 ± 1.9 years and at follow-up (T2) 33.3 ± 2.3 years. Total average treatment time was 3.5 ± 1.4 years. According to the personal adherence cards, the actual daily wearing time ranged between 12 and 14 hours. 50% of all patients (n = 9) had been treated with active removable appliances prior to bionator treatment, and 50% (n = 9) had not received any appliance apart from the bionator.

Nonparametric Friedman's two-way analysis of variance showed significant overall changes from T0 to T2 for all parameters except for lower arch depth and sagittal relationship of left canines (Table 3).

Pairwise comparisons revealed that there was a significant increase in transverse intermolar distance in the upper and lower arches during treatment (T0–T1), whereas the lower intercanine distance did not increase significantly. Arch perimeter was significantly greater after treatment in the upper, but not in the lower dental arch (Table 3). There was a slight, but not significant, decrease in arch depth in the upper arch and an increase in the lower arch. Overjet, overbite, and PAR index showed a significant decrease during bionator treatment (T0–T1). Mandibular incisor irregularity decreased slightly, but not significantly. The sagittal relationship at the first molars improved significantly from 0.5 premolar widths distal occlusion to neutral occlusion at both sides, with a similar, but not significant tendency for the canine relationship on the right side.

In the course of the follow-up period of 20 years (T1–T2), the upper and lower intermolar distance decreased, but not significantly (Table 3). Lower intercanine distance as well as upper and lower arch perimeter decreased significantly with a similar tendency for upper and lower arch depth. There was also a slight, not significant increase in overjet, overbite, and PAR index as well as a significant increase in mandibular incisor irregularity. Sagittal relationships of first molars changed not significantly from neutral occlusion to -0.1 premolar widths mesial occlusion. On the right side, distal occlusion of canines improved slightly toward neutral occlusion with occlusion at the left canines remaining unchanged.

The method error ranged from 0 to 0.8 as calculated by Dahlberg's formula. Intrarater and interrater reliability of dental cast measurements were substantial, with ICC ranging from 0.8 to 1 for all outcome variables.

The aim of this retrospective cast analysis was to evaluate long-term changes of dental parameters after treatment with a functional orthopedic appliance (Balters bionator modified by Ascher) not followed by treatment with fixed appliances. Therefore, dental casts were analyzed before, at the end of bionator treatment, and 20 years after treatment.

The results of the present study showed that overjet was reduced significantly during bionator therapy (T0–T1) as well as sagittal molar and canine relationship were improved toward Class I. Overjet showed a small, not significant increase in the long-term observation (T1–T2), but all parameters remained reasonably stable 20 years after the end of treatment with only arch perimeter and lower intercanine distance decreasing and mandibular incisor irregularity increasing significantly. Several longitudinal studies on physiological development of dental arches and maturation have shown that, without any treatment, overjet decreases only a very small amount.^10–12  Other studies reported similar results on long-term outcomes of bionator treatment based on measurements of dental casts.¹⁵  However, this study was the first to evaluate long-term effects of bionator treatment without follow-up treatment with fixed appliances, thus enabling an assessment of the long-term stability of bionator therapy itself, taking physiological changes due to aging into consideration.

The median PAR index was reduced 63.1% after treatment (T0–T1). Comparing the median of the PAR index before treatment and after long-term observation, there was still a reduction of 33.3%. Francisconi et al. found a higher reduction (81.8%) after treatment with a bionator and fixed appliances¹⁵  and 4.9% relapse 10 years after treatment. Bock et al. reported an increase of the PAR index (+4.8) more than 15 years after treatment with the Herbst appliance.²⁰  In the present study, +5.5 relapse in the median of the PAR index 20 years after treatment was observed. The fact that there was no treatment with fixed appliances and no retention with fixed retainers could explain, why less reduction of the PAR index and more relapse was detected in the current investigation compared to other studies.

Maxillary and mandibular arch perimeter increased during treatment with the increase in the upper arch being significant. This effect was most likely a result of age-dependent physiological development (aging), but there might also have been a small distalizing bionator effect on the upper molars. Long-term, a significant decrease of arch perimeter in both arches was observed, most likely due to age-dependent physiological changes.^11,12 

There were no significant changes in upper and lower arch depth, neither during treatment nor long-term. After bionator treatment, upper arch depth was smaller and lower arch depth higher. This was possibly the result of a distalizing bionator effect on the upper molars and a slight protrusion of the lower incisors. The latter, however, was not significant, most likely due to the covering of the incisors according to Ascher's modification, which is supposed to prevent lower incisors from protruding. A minor decrease of arch depth 20 years later is, according to most authors, a result of physiological processes related to aging.^10–12 

After the first observational period, there was a significant increase in arch width in both arches posteriorly. The gain in transverse width can most likely be attributed to development and growth. Mandibular intercanine distance, by contrast, increased only to a minor degree. Transverse development is associated with the eruption of teeth and mandibular intercanine distance is, most likely, the first transverse distance to remain stable during growth.²¹  In agreement with the findings of Pancherz²²  and others^10–12  intercanine distance decreased significantly over the long-term period (T1–T2). Again, this was most likely related to aging. The current study involved patients, who were only treated with bionators and no fixed appliances. Therefore, no therapeutic expansion of the dental arches was performed at any time, which could otherwise have accounted for this observation due to transverse relapse.

Mandibular incisor irregularity improved slightly during treatment, probably due to additional space in the anterior segment achieved by the physiological increase in transverse widths and the slight protrusion of incisors, which might be attributable to bionator treatment. After the long-term follow-up (T1–T2), however, there was a significant increase in lower incisor irregularity, which was most likely associated with the significant decrease in both lower arch perimeter and intercanine width, in agreement with reports after treatment with the Herbst appliance²²  and with published data on physiological growth changes.^10–12 

Although this study provided data for the first time on long-term effects of bionator treatment without follow-up treatment with fixed appliances, there were some limitations to note. In general, follow-up studies are inherently biased, as it is not possible to locate all potential study participants and not every patient located is willing to participate. Furthermore, patients willing to participate tend to be those satisfied with treatment. Another limitation was the reduced number of study patients available for assessment, which of course limited study power and generalizability. In retrospective, long-term studies, however, this problem is commonly unavoidable due to patients not being available for follow-up examination or heterogeneity of the initially treated patient collective. Since study parameters could not be predetermined prospectively, strict inclusion criteria were required to reduce biasing factors and to achieve a homogeneous patient sample and, thus, sufficient generalizability of results. Additionally, results of the treatment changes and long-term effects of bionator treatment reported in this study need to be interpreted in view of the naturally occurring aging processes of the dentition, which most likely also contributed to treatment effects and long-term stability. Due to ethical reasons, no control group with comparable dental and skeletal configuration at T0, but left untreated, could be analyzed. The matching of patient data to a historical control group as a possible solution can, however, introduce serious bias,²³  thus limiting its usefulness.

Although bionator treatment cannot replace fixed appliances, as many treatment goals can only be achieved with these appliances, poor oral hygiene and development of white spot lesions are a common finding following fixed therapy and related to its duration.^24–26  Bionator treatment prior to fixed orthodontic therapy could thus be a reliable option to reduce treatment time with fixed appliances, as Class I occlusion can already be stably achieved in advance.

• After treatment with a Balters bionator modified by Ascher, overjet and PAR index were significantly reduced and sagittal first molar relationship significantly improved. These treatment results remained reasonably stable in the long term, 20 years after treatment.

• Significant long-term changes such as a decrease in dental arch perimeter, decrease in intercanine distance, as well as an increase in mandibular incisor irregularity, are most likely due to physiological, age-related processes and not associated with the bionator treatment itself.